Metal-alkaline cells are widely used as convenient and efficient sources of stored electrochemical energy. In particular, zinc-alkaline batteries are safe, lightweight, inexpensive, and have a high energy density. Zinc-alkaline batteries are available in the form of both primary (non-rechargeable) and secondary (rechargeable) batteries. Consequently, zinc-alkaline batteries are an extremely popular source of electrochemical energy and are used in a wide variety of applications, ranging from automobiles to portable electrical devices, such as cellular telephones.
In recent years, zinc-air cells have emerged as one of the most popular types of metal-alkaline batteries. Like zinc-metal batteries, zinc-air cells have extremely high energy density, and are safe, lightweight and inexpensive. A zinc-air battery generally comprises an air cathode; a separator film; and an anode mixture comprising an electrolyte and zinc metal particles. A first terminal is electrically connected to the air electrode, and a second terminal is in electrical contact with the anode mixture. The separator film is in contact with both the mixture and the air cathode and separates the two, while allowing ions (i.e., the electrolyte) to travel between the two. The zinc-air battery container also usually has one or more holes to allow air flow from the atmosphere.
Unlike most batteries, a zinc-air battery has only a single consumable electrodexe2x80x94the zinc anode. The zinc anode of a zinc-air cell is typically formed into a gel by adding conventional gelling agents to a mixture comprising an alkaline electrolyte (i.e., KOH), zinc metal particles (i.e., ZnO particles), and a corrosion inhibitor. The gel holds the zinc particles in place, allowing the zinc particles to contact and interact with each other. The cathode usually comprises a layer of active carbon, an oxygen-reducing catalyst, a binder, a metal collector, and a guard layer. Oxygen-reducing catalysts convert oxygen from air into hydroxyl ions, which then oxidize the anode. Electrons are then liberated from the anode.
While zinc-air cells are useful in a wide variety of applications, they also suffer from a number of disadvantages. In particular, zinc-air cells exhibit a high rate of anode corrosion, which over time depletes the battery of stored energy. Typically, the zinc anode in the zinc-air cell reacts with air and other elements in the battery to corrode spontaneously, producing hydrogen as a by-product. Corrosion of zinc-air batteries is exacerbated at elevated storage temperatures. Furthermore, if not permitted to escape from the casing of the cell, the hydrogen gas produced during the corrosion reaction can cause buildup of internal pressure in the cell, and can lead to electrical shorts, swelling, and leaks.
To reduce corrosion rates in zinc-alkaline batteries, various corrosion inhibitors have been tried. For example, mercury has in the past been a widely used corrosion inhibitor. Mercury reduces hydrogen gas production, and thereby reduces the rate of zinc corrosion. As a result, in the presence of mercury the battery life and the stability of the cell are greatly increased. However, because of environmental and safety concerns, mercury is no longer preferable as a corrosion inhibitor.
Numerous other corrosion inhibitors besides mercury, such as metallic materials, organic surfactants, and non-organic materials, have also been used. In the realm of organic surfactant corrosion inhibitors, polyethylene glycol (xe2x80x9cPEGxe2x80x9d) has been discovered to be useful to inhibit corrosion in zinc-alkaline batteries. PEG has a general chemical formula of [HOxe2x80x94(CH2xe2x80x94CH2xe2x80x94O)nxe2x80x94H]. PEG is added to the gelling mixture during the production of the Zn anode and coats the zinc particles, reducing exposure of the Zn metal to electrolytes and thereby inhibiting corrosion.
Although PEG is an effective corrosion inhibitor, it suffers from significant disadvantages. PEG is unstable in the alkaline solution of a zinc-alkaline battery. In addition, PEG does not dissolve well in the alkaline electrolyte emulsions of KOH, water, zinc, and gelling agents used in creating the anode mixture, producing a phase separation. As a result of its low solubility, PEG may deposit on the sides of the mixing container during preparation of the cathode gel mixture, making consistent manufacture of the zinc anode difficult. Moreover, with prolonged storage PEG gradually separates from the zinc particles in the anode, thereby reducing its effectiveness as a corrosion inhibitor.
A further drawback of conventional corrosion inhibitors is that in general, batteries with reduced corrosion rates exhibit reduced electrical capacity and working potential. Although reduced corrosion rates are desirable to enhance battery life, batteries with reduced electrical capacity and working potential are not desirable. Consequently, there is a long-standing need for a corrosion inhibitor for use with metallic-alkaline batteries, specifically with zinc-air batteries, that possess comparable corrosion-inhibiting properties to that of PEG, but which does not share its disadvantages.
The present invention is a method of reducing corrosion in zinc-alkaline battery cells, such-as zinc-air cells, that comprises incorporating derivatives of polyethylene glycol (PEG) having hydrophilic moieties attached to the ends of the PEG chains into the anode of such cells. By incorporating PEG derivatives having hydrophilic moieties attached to the ends of the PEG chains into the anode of such cells, the corrosion rates of the zinc anodes are reduced, and both the storage life of the battery and the electrochemical performance of such batteries are increased. In a preferred embodiment, the corrosion inhibitor is polyethylene glycol, bi-carboxy methyl ether (PEG BCME). The present invention also relates to a zinc alkaline battery containing a zinc anode that incorporates a corrosion inhibitor comprising PEG containing hydrophilic moieties attached to the ends of the PEG chains.
Accordingly, it is an objective of the present invention to provide a metal-alkaline battery cell comprising a cathode including a catalyst and a conductive material; a first terminal electrically connected to the cathode; a mixture comprising an electrolyte, a metal, and a PEG derivative, the electrolyte comprising at least one ion, the PEG derivative having a hydrophilic moiety attached to at least one terminal hydroxyl group of a PEG molecule; a second terminal electrically connected to the mixture; and a separator, the separator being in contact with each of the cathode and the mixture while separating the cathode and the mixture from each other, and allowing the at least one ion in the electrolyte to travel between the mixture and the cathode.
It is another objective of the present invention to provide a zinc-air battery cell comprising a first terminal electrically connected to an air electrode; a mixture comprising an electrolyte, zinc metal particles, and PEG BCME, the electrolyte comprising at least one ion; a second terminal electrically connected to the mixture; and a separator, the separator being in contact with each of the electrode and the mixture while separating the electrode and the mixture from each other, and allowing the at least one ion in the electrolyte to travel between the mixture and the electrode.
It is another objective of the present invention to provide a method of making a metal-air battery cell, comprising the steps of obtaining an air electrode with a first terminal connected thereto; mixing an electrolyte, metal particles, and PEG BCME to create a mixture, the electrolyte comprising at least one ion; electrically connecting a second terminal to the mixture; and positioning a separator between the air electrode and the mixture, the separator being in physical contact with the air electrode and with the mixture while separating the electrode and the mixture from each other, and allowing the at least one ion to travel between the air electrode and the mixture.
The present invention is a metal alkaline battery that incorporates an improved corrosion inhibitor in the anode. Unlike conventional corrosion inhibitors, the corrosion inhibitor of the present invention reduces corrosion and enhances storage life while retaining a high level of electrochemical performance. The corrosion inhibitor comprises PEG modified by addition of hydrophilic moieties at the ends of the PEG chains. In a preferred embodiment, the corrosion inhibitor is polyethylene glycol, bi-carboxy methyl ether (PEG BCME). The present invention is also directed to a method for making a metal alkaline battery that exhibits reduced corrosion, enhanced storage life, and an extended high level of performance. Zn-alkaline batteries containing PEG BCME incur reduced corrosion in comparison to Zn-alkaline batteries containing PEG. It has also been found that zinc-alkaline batteries containing PEG BCME surprisingly do not exhibit the usual reduction in electrical capacity and working potential seen in batteries having reduced corrosion rates. Furthermore, because corrosion of such cells is inhibited in the presence of the corrosion inhibitor of the present invention, hydrogen production is also reduced, and therefore swelling, leaks and electrical shorts are reduced in such zinc-alkaline cells.
The corrosion inhibitor of the present invention is a modified PEG molecule comprising made hydrophilic moieties attached via the hydroxyl groups at the ends of a polyethyl glycol (PEG) chain. The hydrophilic moieties may be any that are effective to neutralize the effects of the alkaline electrolyte solution. Preferably, the hydrophilic moieties are carboxyl (COOxe2x88x92) groups, or more preferably, carboxymethyl (CH3xe2x80x94COOxe2x88x92) groups. Other groups that could be employed include carboxyethyl (CH3xe2x80x94CH2xe2x80x94COOxe2x88x92), carboxypropyl (CH3xe2x80x94CH2xe2x80x94CH2xe2x80x94COOxe2x88x92), and carboxybutyl (CH3xe2x80x94CH2xe2x80x94CH2xe2x80x94CH2xe2x80x94COOxe2x88x92), groups; amino, aminomethyl, aminoethyl, aminopropyl, and aminobutyl groups; and methyl-, ethyl-, propyl-, and butylphosphoesters, and methyl-, ethyl-, propyl-, and butylthiols. Where carboxymethyl groups are attached to the PEG chain, the corrosion inhibitor produced is polyethylene glycol bicarboxy methyl ether (PEG-BCME) (also called polyethylene glycol bis(carboxymethyl)ether). PEG BCME has a general chemical formula of HO2CCH2Oxe2x80x94(CH2xe2x80x94CH2xe2x80x94O)nxe2x80x94CH2CO2H. The value of n in the formula for PEG BCME is preferably in a range between 5 and 50, and the molecular weight is preferably in a range between 200 and 2,000. In a particularly preferred embodiment, n=11; in another particularly preferred embodiment, the PEG BCME has a molecular weight of 600. An appropriate amount of the corrosion inhibitor is added to the zinc anode mixture. The amount of PEG BCME added is preferably sufficient to make a final PEG BCME concentration between about 50 and about 5,000 ppm, and more preferably is in an amount sufficient to make a final PEG BCME concentration between about 200 ppm and about 1,500 ppm.
As noted above, an anode mixture for a metal alkaline battery contains an alkaline electrolyte, usually KOH. In such an anode mixture, the COOxe2x88x92 portions of PEG BCME interact with K+ ions from the KOH salt to form a more stable emulsion than is possible in the presence of PEG. As a result, PEG BCME dissolves more readily in the alkaline electrolyte solution than PEG. Furthermore, unlike PEG, PEG BCME does not precipitate onto the walls of the mixing container during preparation of the anode mixture, leading to a more consistent anode preparation that is suitable for manufacturing.